The present invention is directed to a system for spraying fluent materials, and more particularly to a system which is capable of providing both a continuous spray of fluent material and an instantaneous spray of the material at a predetermined pressure. While not intended to be limited thereto, the present invention will be discussed as it applies to a painting system, particularly a system for painting lines and stripes on roadways.
The major components of a conventional road painting system presently in widespread use are illustrated in FIG. 1. The material to be painted, which can consist of plastic particles, a resin and an aggregate such as sand or glass particles, is placed in a preheater 10 and thoroughly mixed by an agitator 12 within the preheater. A burner 14 located on the underside of the preheater provides sufficient heat to melt the plastic particles and maintain the material in a molten state.
At the beginning of a painting operation, the fluent material in the preheater is transferred into a pressure pot 16, by opening a suitable valve 18 in a feed line 20 connected to the preheater, for example. Sufficient molten material is transferred into the pressure pot as required for the spraying operation, and for large spraying jobs the pot can be filled nearly to capacity. Heat is supplied to the pressure pot by means of a burner 24 to maintain the material in its molten state, and the material in the pot continues to be mixed by means of an agitator 26 disposed within the pot. A cover 22 is placed on the pressure pot and bolted thereto, or otherwise suitably attached, to form an airtight container. After the cover is suitably sealed onto the pressure pot, a valve 28 is opened to supply compressed air from an air compressor 30 to the pot, and thereby pressurize the contents of the material therein. The pressurized material within the pot is then sprayed onto the roadway as desired through the control of a nozzle 32 in fluid communication with the pressure pot.
The pressure pot method of spraying a molten material suffers from a number of disadvantages. One of these is the fact that it is not capable of providing a continuous spraying operation over a long period of time. The length of any one spraying operation is limited by the capacity of the pressure pot. After the pot has been emptied through spraying, its cover must be removed and a new load of molten material from the preheater must be transferred thereto. The cover is then replaced and the pot must be repressurized before the next spraying operation can commence. It will be appreciated that the pressure pot method of spraying necessitates intermittent operation and requires considerable down time of the spraying equipment while the pot is being refilled. This can be a serious consideration for lengthy road painting operations, such as the spraying of a continuous line along a long stretch of highway.
Another drawback of a far more serious nature is the serious safety hazards which the pressure pot method of spraying presents. Due to the high viscosity of the molten plastic material, a substantial pressure must be maintained within the pressure pot. It is not uncommon to repressurize the pot as the level of molten material in the pot decreases. At times, the compressed air introduced into the pressure pot from the air compressor can have oil or other ignitable fluids mixed with it. If the level of molten material in the pot is relatively low, and if the heat supplied by the burner is sufficient to reach the flash point of the oil or other ignitable material in the air, the pressure pot can explode due to the relatively large volume of compressed air within it. Several instances of such explosions, which have resulted in serious injury to the operators of the painting system, have been recorded.
It is therefore an object of the present invention to provide a novel method and apparatus for spraying fluent materials which provides continuous, rather than intermittent, spraying operations.
It is another object of the present invention to provide a novel method and apparatus for spraying fluent materials which eliminates the requirement for compressed air to provide sufficient pressure to spray a highly viscous material, and thereby avoid the safety hazards associated with pressure pot type spraying systems.
In accordance with one aspect of the present invention, the pressure pot is eliminated and a screw conveyor is utilized to transfer the fluent material to be sprayed from a preheater to the spray nozzle. In addition to providing continuous feed of the fluent material from the preheater to the nozzle, the screw conveyor pressurizes the material and eliminates the need for a compressed air source, thereby overcoming the disadvantages associate with the prior art spraying systems.
The use of screw conveyors for spraying systems, particularly painting systems, is known as shown, for example, by U.S. Pat. Nos. 1,726,412 and 4,049,197. Painting systems of the type exemplified by these patents are not suitable for use in applications such as the painting of roadways, since these systems do not maintain the material to be sprayed pressurized at all times. In the painting of dashed lines on roads, for example, it is important that the beginning and ending points of each dash be well defined. In order to obtain such a result, the material to be sprayed must be maintained under pressure so that an instaneous spray of paint at a predetermined pressure can be obtained immediately upon opening the spray nozzle. In the painting systems disclosed in the above-noted patents, the paint is not maintained under pressure between each spray occurrence, and therefore a delay exists between the time when the nozzle is actuated to begin emitting paint and the time when the paint reaches a predetermined spraying pressure. This time delay as the paint pressure builds up results in the production of a dash whose beginning and ending points are not sharply defined. It will be appreciated that the prior art painting systems using a screw conveyor are likewise not acceptable for other types of applications wherein the paint sprayer should provide a precise definition of the painted area.
It is therefore a further object of the present invention to provide a novel method and apparatus for spraying a fluent material which is capable of producing an instantaneous spray of the material at a predeterrmined pressure.
In accordance with this further feature of the present invention, a predetermined torque is maintained on the screw conveyor at all times, to thereby maintain a predetermined pressure on the fluent material at the output end of the conveyor. In addition, the characteristic dimensions of the screw in the conveyor are correlated with the anticipated rate of flow of the fluent material during spraying so that the volume of material displaced by the screw is larger than the expected material usage, to thereby maintain the predetermined pressure on the material during the spraying of the material.